Cancel callback on DOMFileSystem::ScheduleCallback on null context

|execution_context| there may be null, and ScheduleCallback()
should be nop in that case.

Bug: 802373
Change-Id: I6731f9a47f4460439acee6307ec42a5a3e931830
Reviewed-on: https://chromium-review.googlesource.com/881122
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#531403}(cherry picked from commit 44fe59c8dd90d446da8eceda0cb28a5117311be1)
Reviewed-on: https://chromium-review.googlesource.com/936526
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#588}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
diff --git a/third_party/WebKit/Source/modules/filesystem/DOMFileSystem.cpp b/third_party/WebKit/Source/modules/filesystem/DOMFileSystem.cpp
index 5182c15a..ac9f853 100644
--- a/third_party/WebKit/Source/modules/filesystem/DOMFileSystem.cpp
+++ b/third_party/WebKit/Source/modules/filesystem/DOMFileSystem.cpp
@@ -207,6 +207,9 @@
 
 void DOMFileSystem::ScheduleCallback(ExecutionContext* execution_context,
                                      base::OnceClosure task) {
+  if (!execution_context)
+    return;
+
   DCHECK(execution_context->IsContextThread());
 
   std::unique_ptr<int> identifier = std::make_unique<int>(0);